S1AWBM (with March 1 Android security patch level) is out now for most US carriers. The bootloader version is unchanged at 1, so it's possible to revert from AWBM to a previous firmware version. As indicated by the S before the bootloader digit (and confirmed by the release notes), AWBM is only a security update, so there are no feature or performance enhancements. If you have an unlocked phone (e.g. S918U1 rather than S918U), if your carrier hasn't released AWBM yet OTA, you can download it via Frija using the CSC of any carrier who has released it (e.g. TMB), and then install it via Odin.